Absorbent structures containing stiffened fibers and superabsorbent material

ABSTRACT

Absorbent structures having a fluid acquisition/distribution layer with an average dry density of less than about 0.30 g/cc, an average density upon wetting with 1.0% NaCl aqueous solution of less than about 0.20 g/cc, and an average dry basis weight from about 0.001 to about 0.10 g/cm 2  ; and a fluid storage layer positioned beneath the acquisition/distribution layer comprising at least about 15% superabsorbent material. The fluid acquisition/distribution layer comprises from about 50% to 100% chemically stiffened cellulosic fibers and from 0% to about 50% binding means.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of the abandoned applicationhaving U.S. Ser. No. 07/468,549, filed Jan. 23, 1990 in the names ofJeffery T. Cook, Glen R. Lash, Danny R. Moore and Gerald A. Young.

FIELD OF THE INVENTION

This invention relates to absorbent structures using both cellulosicfiber material and superabsorbent material. The absorbent structures canbe used in a variety of absorbent articles such as disposable diapers,adult incontinence pads and briefs and the like which are required tohandle relatively large amounts of discharged body fluids, especiallyrepeated discharges of relatively large amounts of fluid in relativelyshort amounts of time.

BACKGROUND OF THE INVENTION

Absorbent webs which comprise entangled masses of fibers, i.e., fibrouswebs, are well known in the art. Such webs can imbibe liquids, such asdischarged body fluids, both by an absorption mechanism wherein fluid istaken up by the fiber material itself and by a wicking mechanism whereinfluid is acquired by, distributed through and stored in the capillaryinterstices between fibers. One means for improving the absorbencycharacteristics of such fibrous web structures is to incorporate thereinsuperabsorbent material, such as as polymeric gelling material (alsoreferred to as hydrogel-forming material superabsorbent polymers, etc.)which imbibe fluid. The superabsorbent material serves to retain fluidsuch as discharged body liquids. An absorbent structure of this typewherein hydrogel-forming materials in particulate form are incorporatedinto fibrous webs is disclosed in Weisman and Goldman; U.S. Pat. No.4,610,678; Issued Sep. 9, 1986.

The improvement in absorbency provided by incorporation of absorbentgelling materials has permitted the realization of absorbent articlessuch as diapers which employ relatively thin absorbent cores and whichare, therefore, relatively thin products. Thinner diapers are less bulkyto wear and fit better under clothing. They are also more compact in thepackage, making the diapers easier for the consumer to carry and store.Compactness in packaging also results in reduced distribution costs forthe manufacturer and distributor.

One such absorbent core configuration which is useful for use as theabsorbent structure in relatively thin absorbent articles is disclosedin U.S. Pat. No. 4,765,780, issued Aug. 23, 1988 (Angstadt). This patentdiscloses absorbent articles, such as diapers, which have a two layerabsorbent core configuration wherein the core comprises an upper primarylayer and a lower dusting layer. The primary layer is an airlaid web ofhydrophilic fiber material with a substantial amount of absorbentgelling material admixed therewith. The dusting layer compriseshydrophilic fiber material and, preferably, contains no absorbentgelling material.

Another absorbent core configuration is disclosed inWeisman/Houghton/Gellert, U.S. Pat. No. 4,673,402, issued Jun. 16, 1987.This patent discloses absorbent articles having a dual layer absorbentcore. In the dual layer configuration, the core comprises an upperprimary layer which is an airlaid web of hydrophilic fiber material,optionally with a small amount of polymeric gelling agent particlesadmixed therewith. The core also comprises an underlying insert layerwhich is an airlaid mixture of hydrophilic fiber material and asubstantial amount of polymeric gelling agent particles. This insertlayer is generally positioned toward the front of the absorbent articlesuch that more than half of the polymeric gelling agent material in thearticle is found in the front half thereof. Absorbent articles havingthe particular dual layer configuration of the '402 patent can beprepared in the form of especially thin, highly effective, low leakagediaper products.

Notwithstanding the existence of absorbent cores as described above,there remains a need to provide absorbent cores with improved effectiveabsorbent capacity. One way to theoretically do this would be toincrease the level of polymeric gelling material in the absorbent core.Unfortunately, high levels of polymeric gelling material, (especiallylevels in excess of about 15%) in fibrous webs typically used inabsorbent cores tends to induce a phenomena referred to as gel-blocking.Gel-blocking occurs when the polymeric gelling material located inregions first contacted with fluid increase in volume as a consequenceof imbibing the fluid and forming the hydrogel. When polymeric gellingmaterial concentration is too high, the hydrogel can block additionalfluid from reaching other regions of the core having unused absorbentcapacity. The occurrence of gel blocking can lead to leakage duringusage of the absorbent article.

Polymeric gelling materials have been developed which can exhibit areduced tendency to result in gel blocking. Such materials are describedin U.S. Pat. No. Re. 32,649, Apr. 19, 1988, Brandt/Goldman/Inglin.However, these improved polymeric gelling materials, and othersuperabsorbent materials, are subject to performance limitations of theweb of cellulosic fibers in which particles of gelling material aredistributed. In particular, upon initial wetting, the cellulosic fibersbecome highly flexible and the web tends to collapse to a higher densityand, consequently, exhibits smaller average pore size. Whereas, poresize becomes smaller than the pore size in regions of the web not yetwetted, a capillary gradient is created which opposes efficienttransport of fluids to the dry areas of the absorbent article.

Another reason why many absorbent articles such as diapers are subjectto leakage is inability to absorb second and subsequent discharges offluid even if the first fluid discharge has been effectively absorbed.Leakage due to second and subsequent discharges is especially prevalentduring the night, when users commonly experience multiple dischargesbefore being attended to. One reason for the inability of many absorbentarticles to adequately handle multiple discharges of fluid, in additionto the reasons discussed above, is the inability of the absorbent coreto transport discharged fluid away from the region of discharge once theabsorbent capacity of that region has been reached. After a discharge offluid occurs, the fluid tends to remain situated in the region proximateto the discharge. The occurrence of successive voiding of fluid createsa driving force to laterally transport the previous fluid and newlydischarged fluid. However, actual performance of the absorbent articleis limited by the ability to have the fluid transported to the fartherreaches of the core. In this regard, even in the absence of polymericgelling material, the overall absorbent capacity of conventionalabsorbent diaper cores is generally incompletely utilized prior tofailure, i.e., leakage, of the absorbent article.

Yet another reason for leakage in conventional absorbent articles is thepropensity of the cellulosic fibers conventionally utilized for fluidacquisition and distribution to collapse upon wetting, thus impairingpermeability of the structures.

It is an object of this invention to provide superabsorbent-containingabsorbent structures which can circumvent the problems of gel blockingand wet collapse and which can utilize an increased proportion of theirabsorbent capacity.

It is a further object of this invention to providesuperabsorbent-containing absorbent structures which can acquire fluidrapidly in the region of discharge and transport the fluid overrelatively large proportion of the absorbent structure storage area and,additionally, be capable of effectively acquiring and distributingdischarged bodily fluid from second or other successive voiding.

It is yet another object of this invention to provide absorbentstructures which are capable of meeting the objects described abovewhich are of a relatively thin design.

One absorbent structure which has been suggested is described in U.S.Pat. No. 4,935,022, issued Jun. 19, 1990 to Glen R. Lash and Leonard R.Thompson. This patent discloses disposable absorbent articles comprisinga layered absorbent core positioned between a backsheet and a topsheet,wherein the absorbent core comprises an upper layer of stiffened,twisted, curled cellulose fibers and requires from about 3% to 15%, byweight, of large particle absorbent gelling material and a lower layerof stiffened, twisted, curled cellulose fibers and from about 15% to60%, by weight, of absorbent gelling material. The upper layer servesthe principal purpose of acquisition and distribution of bodily fluiddischarges. The stiffened, twisted, curled fibers are highly beneficialin this regard. The lower layer, which is necessarily smaller than theupper layer, is principally for fluid storage.

Another absorbent structure which has been proposed is described in U.S.Pat. No. 4,798,603, S. C. Meyer et al., issued Jan. 17, 1989, titled"Absorbent Article Having a Hydrophobic Transport Layer." As suggestedby the title, this patent describes an absorbent article with ahydrophobic transport layer, made from known hydrophobic syntheticfibers. The transport layer is positioned between a topsheet and anabsorbent body. The absorbent body is necessarily more hydrophilic thanthe transport layer. The purpose of the transport layer is to act as aninsulating layer between the topsheet and the absorbent body, to reduceskin wetness. Regardless of whether the structures described thereinmeet this objective, the hydrophobic nature of the transport layer ofU.S. Pat. No. 4,798,603 would be expected to have limited fluidacquisition and fluid transport properties due, at least in part, to thehydrophobicity of the layer. This would be particularly so for secondand successive fluid discharges after which any optional surfactantshave been washed away.

Notwithstanding the existence of absorbent articles of the typedescribed above, there is a need to identify further improvedconfigurations for absorbent articles which provide improved fluiddistribution and acquisition performance, especially with respect tosuccessive fluid discharges.

Accordingly, the present invention provides improved absorbentstructures, and elements for use therein, as well as absorbent articlesutilizing such structures, utilizing a multiple layer absorbent corethat effectively and efficiently acquires the wearer's discharged bodyfluids upon initial and successive discharges, transports acquiredfluid, from both initial and successive discharges over a relativelylarge proportion of the absorbent structure surface area, and storessuch discharged fluids.

SUMMARY OF THE INVENTION

The present invention provides an absorbent structure, which isparticularly useful as the absorbent core in disposable absorbentarticles such as diapers and incontinence briefs, and which comprises:a) a fluid acquisition/distribution layer having an average dry densityof less than about 0.30 g/cc, an average density upon wetting tosaturation with 1% NaCl aqueous solution, on a dry weight basis, of lessthan about 0.20 g/cc, and an average dry basis weight of from about0.001 to about 0.10 g/cm² ; and a fluid storage layer, positionedbeneath the acquisition/distribution layer. The acquisition/distributionlayer comprises a web of from about 50% to 100%, by weight, chemicallystiffened cellulosic fibers and from 0% to about 50%, by weight, of abinding means. The binding means can be used to increase physicalintegrity of the web to facilitate processing and/or improve in-useperformance, and/or increase effective average inter-fiber pore size ofthe web. As used herein, binding means refers to means incorporatedintegral to the layer of stiffened fibers, such as (but not limited to)nonstiffened cellulosic materials, synthetic fibers, chemical additives,and thermoplastic polymers. Tissue envelopes and other scrim external tothe acquisition/distribution layer can also be used to enhance physicalintegrity in combination with, or in place of, said binding means.

The storage layer comprises at least about 15%, by weight, ofsuperabsorbent material and from 0% to about 85% of a carrier means forthe superabsorbent material. The fluid acquisition/distribution layershould contain no more than about 6.0% of superabsorbent material.Preferably, the acquisition/distribution layer will be substantiallyfree of superabsorbent material. For purposes herein, "substantiallyfree" of superabsorbent material means less than about 2.0%, preferablyless than about 1.0%, more preferably zero or essentially zero percentsuperabsorbent material. As used herein, "essentially zero" percentsuperabsorbent material means low amounts (less than about 0.5%) ofsuperabsorbent material present in the acquisition/distribution layerincidental to the contact or close proximity of thesuperabsorbent-containing storage layer with theacquisition/distribution layer.

The fluid acquisition/distribution layer has a top surface area which isat least 15% of the top surface area of the fluid storage layer, butwhich is smaller than the top surface area of the fluid storage layer.The acquisition/distribution layer is preferably positioned relative tothe fluid storage such that in the unfolded planar configuration of thearticle none of its surface area extends beyond the boundaries of thetop surface area of the fluid storage layer. More preferably theacquisition/distribution layer has a top surface area which is fromabout 15% to about 95%, most preferably from about 25% to about 90%, ofthe top surface area of the fluid storage layer.

The absorbent structure can be advantageously utilized as the absorbentcore in absorbent articles, e.g., disposable diapers and incontinencebriefs, which also comprise a fluid pervious topsheet and a fluidimpervious backsheet affixed to the topsheet, wherein the absorbent coreis disposed therebetween. The absorbent core is positioned such that theacquisition/distribution layer is located between the topsheet and thestorage layer, and the storage layer is located between theacquisition/distribution layer and the backsheet.

The superabsorbent material used in the storage layer has an AbsorbentCapacity of at least about 10 grams of Synthetic Urine (1.0% NaClaqueous (distilled water) solution) per gram of superabsorbent material,measured according to the test procedure hereinafter described. Suitablesuperabsorbent material categories include polymeric absorbent gellingmaterials, typically utilized in the form of discrete particles, andsuperabsorbent fibers, such as acrylate grafted fibers andsuperabsorbent modified cellulosic fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a perspective view of a diaper with an absorbent corehaving the multiple layer configuration of the present invention. Theabsorbent core shown has a rectangular-shaped acquisition/distributionlayer and an hour glass-shaped storage layer.

FIG. 2 represents a perspective view of a diaper structure similar toFIG. 1, but wherein the storage layer has a modified hour-glass shape.

FIG. 3 represents a direct view of an absorbent core useful for diaperapplications, such as in FIGS. 1 and 2, wherein the core has a modifiedhour glass-shaped storage core and a similar hour glass-shapedacquisition/distribution layer.

DETAILED DESCRIPTION OF THE INVENTION

The absorbent structures of the present invention can be utilized indisposable products which are capable of absorbing significantquantities of body fluids, such as urine and water in body wastes. Sucharticles may be prepared in the form of disposable diapers, adultincontinence briefs, adult incontinence pads and the like.

The absorbent articles herein generally comprise three basic structuralcomponents. One such component is a liquid impervious backsheet. On topof this backsheet is disposed an absorbent core which itself comprisestwo distinct layers, and which includes a superabsorbent material in oneof the layers. On top of this absorbent core and joined to the backsheetis a water pervious topsheet. The topsheet is the element of the articlewhich is placed next to the skin of the wearer. As used herein, the term"joined" encompasses configurations whereby the topsheet is directlyjoined to the backsheet by affixing the topsheet directly to thebacksheet, and configurations whereby the topsheet is indirectly joinedto the backsheet by affixing the topsheet to intermediate members whichin turn are affixed to the backsheet. Preferably, the topsheet andbacksheet are joined directly at the diaper periphery by adhesive orother attachment means known in the art.

Especially preferred absorbent articles of this invention are disposablediapers. Articles in the form of disposable diapers are fully describedin Duncan and Baker, U.S. Patent No. Re. 26,151, Issued Jan. 31, 1967;Duncan, U.S. Pat. No. 3,592,194, Issued Jul. 13, 1971; Duncan andGellert, U.S. Pat. No. 3,489,148, Issued Jan. 13, 1970; and Buell, U.S.Pat. No. 3,860,003, Issued Jan. 14, 1975; which patents are incorporatedherein by reference. A preferred disposable diaper for the purpose ofthis invention comprises an absorbent core; a topsheet superposed orco-extensive with one face of the core, and a liquid imperviousbacksheet superposed or co-extensive with the face of the core oppositethe face covered by the topsheet. Both the backsheet and the topsheetmost preferably have a width greater than that of the core therebyproviding side marginal portions of the backsheet and topsheet whichextend beyond the core. Frequently the backsheet and the topsheet willbe fused together in these side marginal portions. The diaper ispreferably constructed in a shaped configuration such as, but notlimited to, an hourglass shape.

The backsheet of the articles herein can be constructed, for example,from a thin, plastic film of polyethylene, polypropylene, or otherflexible moisture impeding material which is substantially waterimpervious. Polyethylene, having an embossed caliper of approximately1.5 mils, is especially preferred.

The topsheet of the article herein can be made in part or completely ofsynthetic fibers or films comprising such materials as polyester,polyolefin, rayon, or the like, or of natural fibers such as cotton. Innonwoven topsheets, the fibers are typically bound together by a thermalbinding procedure or by a polymeric binder such as polyacrylate. Thissheet is substantially porous and permits a fluid to readily passtherethrough into the underlying absorbent core.

Another suitable type of topsheet comprises the topsheets formed fromliquid impervious polymeric material such as polyolefins. Such topsheetscan have tapered capillaries of certain diameter and taper positioned inthe topsheet to permit flow of discharged fluid through the topsheetinto the underlying absorbent core of the article.

The topsheets used in the articles of the present invention should berelatively hydrophobic in comparison with the absorbent core of saidarticles. Topsheet construction is generally disclosed in Davidson, U.S.Pat. No. 2,905,176, Issued Sep. 22, 1959; Del Guercio, U.S. Pat. No.3,063,452, Issued Nov. 13, 1962; Holliday, U.S. Pat. No. 3,113,570,Issued Dec. 10, 1963, and Thompson, U.S. Pat. No. 3,929,135; Issued Dec.30, 1975; which patents are incorporated herein by reference. Preferredtopsheets are constructed from polyester, rayon, rayon/polyester blends,polyethylene or polypropylene. The topsheet can be treated withsurfactant to make it more wettable and therefore relatively lesshydrophobic, to thereby increase fluid flow through it at least uponinitial wetting. However, the topsheet should still be more hydrophobicthan the absorbent article element which receives fluids after passingthrough the topsheet.

An absorbent core, which is preferably flexible, is positioned betweenthe elongated backsheet and the topsheet to form the absorbent articlesherein. This core essentially comprises both an upper fluidacquisition/distribution layer and a lower fluid storage layer. Itshould be understood that for purposes of this invention these two typesof layers refer merely to the upper and lower zones of the absorbentcore and are not necessarily limited to single layers or sheets ofmaterial. Thus both the fluid acquisition/distribution layer and thefluid storage layer may actually comprise laminates or combinations ofseveral sheets or webs of the requisite type of materials as hereinafterdescribed. The storage layer can comprise a single sheet of essentially100% superabsorbent material, as will be hereinafter described. As usedherein, the term "layer" includes the terms "layers" and "layered." Forpurposes of this invention, it should also be understood that the term"upper" refers to the layer of the absorbent core which is nearest toand faces the article topsheet; conversely, the term "lower" refers tothe layer of the absorbent core which is nearest to and faces thearticle backsheet.

Optionally, a fluid pervious sheet (e.g., a tissue sheet) or other scrimis positioned between the acquisition/distribution layer and the storagelayer to increase integrity of the acquisition/distribution layer duringprocessing and/or use. Such sheet or scrim can envelope all or part ofacquisition/distribution layer, or simply be positioned as describedabove without necessarily enveloping the acquisition/distribution layer.Also, optionally, the superabsorbent material-containing storage layercan be enveloped with a fluid pervious sheet, such as a tissue papersheet, to obviate user concerns with loose superabsorbent material.Since sheets of this type are fluid pervious, they will not adverselyaffect the unimpeded fluid communication to be maintained between theacquisition/distribution and storage layers.

Acquisition/Distribution Layer

One essential element of the absorbent structures hereof is an upperfluid acquisition/distribution layer which comprises a combination of ahydrophilic fibrous material, described more fully hereinafter. Thisfluid acquisition/distribution layer serves to quickly collect andtemporarily hold discharged body fluid. A portion of discharged fluidmay, depending upon the wearer's position, permeate theacquisition/distribution layer and be absorbed by the storage layer inthe area proximate to the discharge. However, since fluid is typicallydischarged in gushes, the storage layer in such area may not absorb thefluid as quickly as it is discharged. Therefore, the upperacquisition/distribution layer hereof also facilitates transport of thefluid from the point of initial fluid contact to other parts of theacquisition/distribution layer. In the context of the present invention,it should be noted that the term "fluid" means "liquid."

As previously noted, the fluid acquisition/distribution layer is a webcomprising stiffened cellulosic fibers. The acquisition layer comprisesfrom about 50% to 100% of these fibers and from 0% to about 50% of abinding means. Suitable binding means are discussed below.

The fluid distribution function of the acquisition/distribution layer isof particular importance in order to more fully utilize the capacity ofthe storage section. The presence of substantial amounts ofsuperabsorbent materials in the acquisition/distribution layer whichswell upon contact with fluids is believed to adversely affect thisfunction of the acquisition/distribution layer.

A variety of other factors relating to the fluidacquisition/distribution layer of the absorbent structures herein can beof importance in determining the effectiveness of the resultingabsorbent articles. These include shape, basis weight, density,permeability, capillarity and wicking ability, the type and structuralintegrity, and character of the fibrous material utilized. As indicated,the acquisition/distribution layer of the core is preferably elongated.For purposes of this invention, this means that theacquisition/distribution layer, like the storage layer, is elongated ifit is of unequal length and width in the unfolded, flat configuration.The acquisition/distribution layer in the unfolded configuration can beof any desired shape, for example, rectangular, trapezoidal, oval,oblong or hourglass-shaped. The shape of the upper fluidacquisition/distribution layer of the core can, but need notnecessarily, correspond to the general shape of the storage layer. Thetop surface area of the acquisition/distribution layer will preferablyrange from about 25% to about 90% of the top surface area of the storagelayer, and also preferably will not extend beyond the edge of thestorage layer at any outer boundary. The acquisition/distribution layerwill typically have top surface area less than about 80% of that of thestorage layer.

Preferably, there is a margin from the edge of theacquisition/distribution layer to the edge of the storage layer of atleast about 0.5 cm, preferably at least about 1.25 cm, in the regionsproximate to where fluid is discharged during use. In diapers, thiswould correspond, for example, to the crotch region 115 of FIG. 2,particularly at the narrowest part of the storage core 106 in thecentral region 115. Additionally, especially for absorbent articles tobe worn by males, such a margin is maintained in the front waist region,exemplified as 112 in FIG. 2, which area is to be worn on the front ofthe wearer.

The fluid acquisition/distribution layer will generally have an averagedry density of less than about 0.30 g/cm³, measured prior to use, and anaverage density upon wetting to saturation with Synthetic Urine (1.0%NaCl aqueous solution, with distilled water), on a dry weight basis, ofless than about 0.20 g/cm³, preferably less than about 0.15 g/cm³. Also,preferably, the average dry density and density upon wetting tosaturation are between about 0.02 g/cm³ and 0.20 g/cm³, more preferablybetween about 0.02 g/cm³ and about 0.15 g/cm³. The average dry basisweight of the acquisition/distribution layer of the absorbent core willtypically range from about 0.001 to about 0.10 g/cm², preferably fromabout 0.01 to about 0.08 g/cm², more preferably from about 0.015 toabout 0.04 l g/cm². Unless specifically indicated, all basis weights anddensity values are calculated on a dry basis (at equilibrium moisturelevels no greater than about 6%). Density and basis weight can besubstantially uniform although nonuniform density and/or basis weight,and density and/or basis weight gradients, are meant to be encompassedherein. Thus, the acquisition/distribution layer can contain regions ofrelatively higher or relatively lower density and basis weight,preferably not exceeding the foregoing ranges. Average dry density andaverage dry density upon wetting to saturation with Synthetic Urine(1.0% NaCl aqueous solution, with distilled water) values are calculatedfrom basis weight of the dry layer and layer caliper. Dry caliper andcaliper upon wetting to saturation are measured under a confiningpressure of 0.2 psi (1.43 kPa). Average density upon wetting tosaturation is calculated from the dry basis weight and saturationcaliper. The saturation caliper is measured after the layer is saturated(under unrestrained conditions) with the 1.0% NaCl aqueous solution andallowed to equilibrate.

The acquisition/distribution layer of the absorbent structures hereinessentially comprises a web of hydrophilic chemically stiffenedcellulosic fibers. These cellulosic fibers are typically wood pulpfibers which have been stiffened with an intrafiber chemical stiffeningagent.

The fluid acquisition/distribution layer should contain no more thanabout 6.0% of superabsorbent material. Preferably, theacquisition/distribution layer will be substantially free ofsuperabsorbent material. For purposes herein, "substantially free" ofsuperabsorbent material means less than about 2.0%, preferably less thanabout 1.0%, more preferably zero or essentially zero percentsuperabsorbent material. As used herein, "essentially zero" percentsuperabsorbent material means low amounts (less than about 0.5%) ofsuperabsorbent material present in the acquisition/distribution layerIncidental to the contact or close proximity of thesuperabsorbent-containing storage layer with theacquisition/distribution layer.

If present in the acquisition/distribution layer, especially if presentin amounts greater than about 2.0%, superabsorbent material in the formof particles of absorbent gelling material may be of relatively largediameter (e.g., from about 400 to about 700 microns in mass medianparticle size). Superabsorbent particles having a mass median particlesize less than 400 microns may also be employed.

As discussed above, the articles of the present invention employchemically stiffened fibers. As used herein, the term "chemicallystiffened fibers" means any fibers which have been stiffened by chemicalmeans to increase stiffness of the fibers under both dry and aqueousconditions. Such means include the addition of chemical stiffeningagents which, for example, coat and/or impregnate the fibers. Such meansalso include the stiffening of the fibers by altering the chemicalstructure of the fibers themselves, e.g., by cross-linking polymerchains.

For exemplary purposes, polymeric stiffening agents which can coat orimpregnate cellulosic fibers include: cationic modified starch havingnitrogen-containing groups (e.g., amino groups) such as those availablefrom National Starch and Chemical Corp., Bridgewater, N.J., USA; latex;wet strength resins such as polyamide-epichlorohydrin resin (e.g.,Kymene™ 557H, Hercules, Inc. Wilmington, Del. USA), polyacrylamide resin(described, for example, in U.S. Pat. No. 3,556,932 issued Jan. 19, 1971to Coscia, et al.; also, for example, the commercially availablepolyacrylamide marketed by American Cyanamid Co., Stanford, Conn., USA,under the tradename Parez™ 631 NC); urea formaldehyde and melamineformaldehyde resins, and polyethylenimine resins. A general dissertationon wet strength resins utilized in the paper art, and generallyapplicable herein, can be found in TAPPI monograph series No. 29. "WetStrength in Paper and Paperboard", Technical Association of the Pulp andPaper Industry (New York, 1965).

The fibers utilized in the structures herein can also be stiffened bymeans of chemical reaction. For example, crosslinking agents can beapplied to the fibers which, subsequent to application, are caused tochemically form intra-fiber crosslink bonds. These crosslink bonds canincrease stiffness of the fibers. Whereas the utilization of intrafibercrosslink bonds to chemically stiffen the fibers is preferred, it is notmeant to exclude other types of reactions for chemical stiffening of thefibers.

Fibers stiffened by crosslink bonds in individualized (i.e., fluffed)form are disclosed, for example, in Bernardin, U.S. Pat. No. 3,224,926,Issued Dec. 21, 1965; Chung, U.S. Pat. No. 3,440,135, Issued Apr. 22,1969; Chatterjee, U.S. Pat. No. 3,932,209, Issued Jan. 13, 1976 andSangenis et al., U.S. Pat. No. 4,035,147, Issued Jul. 12, 1977. Morepreferred fibers are disclosed in Dean et al., U.S. Pat. No. 4,822,453,issued Apr. 18, 1989, Dean et al., U.S. Pat. No. 4,888,093, issued Dec.19, 1989, and Moore et al., U.S. Pat. No. 4,898,642, issued Feb. 6,1990. All of these patents are incorporated herein by reference. Inaddition to being hydrophilic, these stiffened fibers remain stiff evenupon wetting; thus webs made from them do not collapse, as do webs madefrom conventional unstiffened fibers when wet. This provides improvedability to acquire and distribute fluids in second and subsequentdischarges.

In the more preferred stiffened fibers, chemical processing includesintrafiber crosslinking with crosslinking agents while such fibers arein a relatively dehydrated, defibrated (i.e., individualized), twisted,curled condition. Suitable chemical stiffening agents include monomericcrosslinking agents including, but not limited to, C₂ -C₈ dialdehydesand C₂ -C₈ monoaldehydes having an acid functionality can be employed toform the crosslinking solution. These compounds are capable of reactingwith at least two hydroxyl groups in a single cellulose chain or onproximately located cellulose chains in a single fiber. Suchcrosslinking agents contemplated for use in preparing the stiffenedcellulose fibers include, but are not limited to, glutaraldehyde,glyoxal, formaldehyde, and glyoxylic acid. Other suitable stiffeningagents are polycarboxylates, such as citric acid. The polycarboxoxylicstiffening agents and a process for making stiffened fibers from themare described in U.S. Ser. No. 596,606, filed Oct. 17, 1990,incorporated by reference herein. The effect of crosslinking under theseconditions is to form fibers which are stiffened and which tend toretain their twisted, curled configuration during use in the absorbentarticles herein. Such fibers, and processes for making them aredescribed in the above incorporated patents.

The preferred stiffened fibers are twisted and curled can be quantifiedby referencing both a fiber "twist count" and a fiber "curl factor". Asused herein, the term "twist count" refers to the number of twist nodespresent in a certain length of fiber. Twist count is utilized as a meansof measuring the degree to which a fiber is rotated about itslongitudinal axis. The term "twist node" refers to a substantially axialrotation of 180° about the longitudinal axis of the fiber, wherein aportion of the fiber (i.e., the "node") appears dark relative to therest of the fiber when viewed under a microscope with transmitted light.The twist node appears dark at locations wherein the transmitted lightpasses through an additional fiber wall due to the aforementionedrotation. The distance between nodes corresponds to an axial rotation of180°. The number of twist nodes in a certain length of fibers (i.e., thetwist count) is directly indicative of the degree of fiber twist, whichis a physical parameter of the fiber. The procedures for determiningtwist nodes and total twist count are described in the hereinbeforereferenced U.S. Pat. No. 4,898,642.

The preferred stiffened cellulose fibers will have an average dry fibertwist count of at least about 2.7, preferably at least about 4.5 twist,nodes per millimeter. Furthermore, the average wet fiber twist count ofthese fibers should preferably be at least about 1.8, preferably atleast about 3.0, and should also preferably be at least about 0.5 twistnodes per millimeter less than the average dry fiber twist count. Evenmore preferably, the average dry fiber twist count should be at leastabout 5.5 twist nodes per millimeter, and the average wet fiber twistcount should be at least about 4.0 twist nodes per millimeter and shouldalso be at least 1.0 twist nodes per millimeter less than its averagedry fiber twist count. Most preferably, the average dry fiber twistcount should be at least about 6.5 twist nodes per millimeter, and theaverage wet fiber twist count should be at least about 5.0 twist nodesper millimeter and should also be at least 1.0 twist nodes permillimeter less than the average dry fiber twist count.

In addition to being twisted, the preferred fibers used in theacquisition/distribution layer of the absorbent structure are alsocurled. Fiber curl may be described as the fractional shortening of thefiber due to kinks, twists, and/or bends in the fiber. For the purposesof this invention, fiber curl is measured in terms of a two dimensionalplane. The extent of fiber curling can be quantified by referencing afiber curl factor. The fiber curl factor, a two dimensional measurementof curl, is determined by viewing the fiber in a two dimensional plane.To determine curl factor, the projected length of the fiber as thelongest dimension of a two dimensional rectangle encompassing the fiber,L_(R), and the actual length of the fiber, L_(A), are both measured. Thefiber curl factor can then be calculated from the following equation:

    Curl Factor=(L.sub.A /L.sub.R)-1.

An image analysis method that can be utilized to measure L_(R) and L_(A)is described in U.S. Pat. No. 4,898,642. Preferably the fibers utilizedin the layers of the absorbent core herein will have a curl factor of atleast about 0.30, and more preferably will have a curl factor of atleast about 0.50.

The degree of stiffening, dependent upon the type and amount ofstiffening agent (i.e., crosslinking agent) used, the degree ofdehydration of the fibers during curing of the crosslinking agent, andthe curing time and conditions, affect the ability of the fiber to takeup fluid and the tendency of the fiber to swell.

The fiber stiffness as it relates to resistance to fiber wall swellingcan be quantified by referencing the water retention value (WRV) of thestiffened cellulosic fibers used in the absorbent articles herein. WRVis a measure of the amount of water retained by a mass of fibers aftersubstantially all of the interfiber water has been removed. Anotherparameter which can be used to characterize the nature of the stiffenedfibers formed by crosslinking fibers in relatively dehydrated form isthat of alcohol retention value (ARV). ARV is a measure of the extent towhich a fluid, e.g., isopropyl alcohol, which does not inducesubstantial fiber swelling, is taken up by the stiffened fibers. The ARVof the stiffened fibers is directly related to the extent that thefibers were swollen with the solution of crosslinking agent during thestiffening procedure. Relatively higher ARVs mean that the fibers weregenerally swollen to a relatively greater extent during crosslinking.Procedures for determining WRV and ARV are described in U.S. Pat. No.4,898,642.

The WRV for the stiffened, twisted, curled fibers used in the presentinvention will preferably range between about 28% and about 50%. In morepreferred embodiments, the WRV of the fibers can range from about 30% to45%. Fibers having a WRV within these ranges are believed to provide anoptimal balance of swelling-induced untwisting and fiber stiffness.

The stiffened cellulose fibers preferred for use herein are those whichhave an ARV (isopropol alcohol) of less than about 30%. The limitationthat such fibers have an ARV (isopropol alcohol of less than about 30%is indicative of the relatively dehydrated, unswollen state of thesefibers during the stiffening process. More preferably, the ARV(isopropol alcohol) of the fibers useful herein will be less than about27%.

The stiffened cellulose fibers herein having the preferred twist count,curl factor, WRV and ARV characteristics hereinbefore set forth, can beprepared by internally crosslinking such fibers in relatively dehydratedform while or after such fibers are being or have been dried anddefibrated (i.e., "fluffed") as described in U.S. Pat. No. 4,898,642. Itis not, however, meant to necessarily exclude other hydrophilic,chemically stiffened fibers from this invention, such other fibers beingdescribed in (but not limited to) the previously Incorporated U.S. Pat.No. 3,224,926, 3,440,135, 4,035,147, and 3,932,209.

A characteristic of stiffened fibers, particularly the twisted, curledstiffened fibers is their ability to partially untwist and uncurl uponwetting. Thus, when formed into webs of sufficient density, the webs canexpand upon wetting to an equilibrium wet density, which, whencalculated, on a dry fiber density, is less than the average dry density(prior to wetting). This accounts for the average dry densities of up toabout 0.30 g/cm³ described above, in conjunction with lower averagedensities upon wetting to saturation. Such webs which can expand uponwetting are described in U.S. Pat. No. 4,822,453. To the extent that itis desired to utilize this characteristic in absorbent article design,those of ordinary skill in the art will be able to adjust the relativeamount of stiffening agent used, and the extent to which twist and curlin the stiffened fibers is imparted, to achieve the desired amount ofexpansion upon wetting.

The stiffened cellulosic fibers can be provided in web form by varioustechniques, including airlaying and wetlaying.

Airlaid Webs

The stiffened cellulosic fibers can be airlaid to form the web of adesired density and basis weight. The stiffened fibers for use in thepresent invention can be airlaid according to techniques well known tothose skilled in the art of airlaying cellulosic fibers. In general,airlaying can be effected by metering an air flow containing the fibers,in substantially dry condition, onto a wire screen and, optionally,compressing the resulting web to the desired density. Alternately, thefibers can be airlaid to the desired density without compression. Theairlaid web will comprise at least about 50% of stiffened cellulosicfibers, as described above, and can comprise up to and including 100% ofsaid fibers. The web can optionally contain binding means, such asdescribed below, or other optional components, such as or ingredientsmodifying fluid handling properties (e.g., hydrophilic surface activeagents), and the like.

Wetlaid Webs

In another embodiment, the stiffened cellulosic fibers, rather thanbeing airlaid to form the web, are wetlaid. The wetlaid webs comprisefrom about 50% to 100% of the stiffened fibers and from 0% to about 50%of a binding means for increasing physical integrity of the web, tofacilitate processing in the wet and/or dry state, and to provideincreased integrity upon wetting of the web during use. Preferably, thewetlaid webs will comprise at least about 2% of a fibrous binding meansor high surface area cellulose binding means (hereafter described).Chemical additives can also be used as binding means, and areincorporated into the acquisition/distribution layer at levels typicallyof about 0.2% to about 2.0%, dry web weight basis.

Techniques for wetlaying cellulosic fibrous material to form sheets suchas dry lap and paper are well known in the art. These techniques aregenerally applicable to the wet-laying of the stiffened fibers to formwetlaid sheets useful in the absorbent structures of this invention.Suitable wetlaying techniques include handsheeting, and wetlaying withthe utilization of papermaking machines as disclosed, for instance, byL. H. Sanford et al. in U.S. Pat. No. 3,301,746. Due to the behavior ofstiffened fibers, particularly their tendency to flocculate in aqueousslurries, certain processing modifications, hereafter described, arepreferably implemented when wetlaying with papermaking machines. Ingeneral, wetlaid webs can be made by depositing an aqueous slurry offibers on to a foraminous forming wire, dewatering the wetlaid slurry toform a wet web, and drying the wet web. Preferably, the aqueous slurriesof fibers for wetlaying will have a fiber consistency of between about0.05% and about 2.0%, preferably between about 0.05% and about 0.2%,total slurry weight basis. Deposition of the slurry is typicallyaccomplished using an apparatus known in the art as a headbox. Theheadbox has an opening, known as a slice, for delivering the aqueousslurry of fibers onto the foraminous forming wire. The foraminousforming wire is often referred to in the art as a Fourdrinier wire. TheFourdrinier wire can be of construction and mesh size used for dry lapor other papermaking processing. Preferably, mesh sizes of about 70 toabout 100 (Tyler standard screen scale) are used. (All mesh sizesreferred to herein shall be based upon the Tyler standard screen scale,unless otherwise specifically indicated.) Conventional designs ofheadboxes known in the art for drylap and tissue sheet formation may beused. Suitable commercially available headboxes include, for example,fixed roof, twin wire, and drum former headboxes. Once formed, the wetweb is dewatered and dried. Dewatering can be performed with suctionboxes or other vacuum devices. Typically, dewatering increases the fiberconsistency to between about 8% and about 45%, total wet web weightbasis, preferably between about 8% and about 22%. Dewatering toconsistencies above about 22% may require wet-pressing and is lesspreferred. After dewatering, the web can be, but is not necessarily,transferred from the forming wire to a drying fabric which transportsthe web to drying apparatuses. The drying fabric is preferably coatsetthan the forming wire, for increased drying efficiency. The dryingfabric preferably has about 30% to about 50% open area and about 15% toabout knuckle area, such as a 31×25 3S (satin weave) fabric that hasbeen sanded to increase the knuckle area to within the preferred range.Wet microcontraction is preferably implemented during transfer from theforming wire to the fabric. Wet microcontraction can be accomplished byrunning the forming wire at a speed which is from about 5% to about 20%faster than the speed at which the fabric is being run. Drying can beaccomplished with a thermal blow-through dryer or vacuum device such asa suction box, although thermal blow-through drying is preferred. Thewetlaid webs are preferably dried to completion (generally to fiberconsistency between about 90% and about 95% by the thermal blow-throughdryers. Blow-through drying is believed to efficiently dry webs of thestiffened fibers due to the high void volume of the webs. Steam drumdrying apparatus known in the art, such as Yankee drum dryers, can beused but are less preferred. Drum dryers are believed to be lessefficient for drying webs of the stiffened fibers and can also compactthe webs. The dried webs are preferably not creped.

As an alternative to drying as described above, the dewatered web can beremoved from the forming wire placed on a drying screen and dried(unrestrained) in a batch drying process by, for example, a thermal blowthrough dryer or a forced convection steam heated oven.

The stiffened fibers have the tendency to flocculate, or form clumps, inaqueous solution. In order to inhibit flocculation, the aqueous slurryshould be pumped to the headbox at a linear velocity of at least about0.25 m/sec. Also, it is preferred that the linear velocity of the slurryupon exit from the headbox slice is from about 2.0 to about 4.0 timesthe velocity of the forming wire. Another method for reducingflocculations of fibers in a wetlaying process is described in U.S. Pat.No. 4,889,597, issued Dec. 26, 1989, incorporated herein by reference,wherein jets of water are directed at the wetlaid fibers just afterdeposition on the forming wire.

Binding Means

Relative to conventional non-stiffened cellulosic fibers, thecrosslinked, twisted, stiffened fibers as described above form lowertensile strength sheets, particular in the undried condition. Therefore,in order to facilitate processing and to increase the integrity of thewebs, particularly for wetlaid webs (although binding means can also beused with airlaid webs), a binding means can be integrally incorporatedinto or onto the web. This can be done by adding the binding means tothe fibers prior to web formation (wetlaid or airlaid web formations),by applying the binding means (e.g., chemical additive binding means) toa wetlaid web after deposition on the forming wire and before drying, byapplying binding means to a dry web (wetlaid), or a combination thereof.

Suitable binding means for addition to the stiffened cellulosic fibersprior to formation of the wet web from a pulp slurry include, but arenot limited to, a variety of cellulosic and synthetic fibrous materials.Such material include nonstiffened cellulosic fibers (i.e., conventionalcellulosic pulp fibers), highly refined, nonstiffened, cellulosic fiberswhich are refined to Canadian Standard Freeness (CSF) of less than about200 CSF, more preferably from about 100 CSF to about 200 CSF (highlyrefined fibers being referred to herein as "crill", and high surfacearea cellulosic material such as expanded cellulose fibers (hereinafterdescribed).

Various types of synthetic fibrous material can be used in the syntheticfiber binding means. For the purposes hereof, the use of "syntheticfibrous materials" as a binding means shall refer to the utilization ofsuch fibrous materials, in the final product, in fibrous form.(Preferably, the synthetic fibers are of at least staple length, i.e.,the fibers preferably having an average length of at least about 1.5cm). Any type of fibrous material which is suitable for use inconventional absorbent products is believed to be suitable for use inthe acquisition/distribution web of the present invention. Specificexamples of such fibrous material include modified cellulose fibers,rayon, polyester fibers such as polyethylene terephthalate (DACRON),hydrophilic nylon (HYDROFIL) and the like. Other fibers useful includecellulose acetate, polyvinyl fluoride, polyvinylidene chloride,acrylics, polyvinyl acetate, polyamides (such as nylon), bicomponentfibers, tricomponent fibers, mixtures thereof, and the like. Hydrophilicfibrous materials are preferred. Examples of suitable hydrophilicfibrous materials include hydrophilized hydrophobic fibers, such assurfactant-treated or silica-treated thermoplastic fibers derived, forexample, from polyolefins such as polyethylene or polypropylene,polyacrylics, polyamides, polystyrenes, polyurethanes and the like.Hydrophobic synthetic fibers can also be used, but are less preferred.Such synthetic fibers that can be added to the web and utilized in thefinal web product in fibrous form include rayon, polyethylene,polypropylene, etc. Such fibers, when of a hydrophobic nature, arepreferably present in quantities of less than about 30%, total webweight basis, such that the web remains substantially hydrophilic.Conventionally, nonstiffened fibers, crill, and synthetic fibers canalso be used in airlaid webs.

In one preferred embodiment wherein the acquisition/distribution layeris made by a wetlaying process, the web comprises from about 85% toabout 95% of the stiffened cellulosic fibers and from about 5% to about15% of crill, preferably from about 90% to about 95% of the stiffenedfibers and from about 5% to about 10% of crill, most preferably about92% of stiffened fibers and about 8% crill. Suitable cellulosic fibersfor use as crill include chemically pulped wood fibers, includingsoftwood and hardwood pulp fibers, preferably southern softwood fibers(e.g., Foley Fluff, The Procter & Gamble Cellulose Co., Memphis, Tenn.,USA). All percentages of web components referred to herein, unlessotherwise expressly stated, are on a dry web total weight basis.

In another embodiment, the acquisition/distribution layer comprises thestiffened fibers and up to about 25% of high surface area cellulosicmaterial such as expanded cellulose fibers. Preferably, theacquisition/distribution layer comprising a web of wetlaid stiffenedfibers and high surface area cellulose will comprise from about 85% toabout 98% of the stiffened fibers, preferably from about 90% to about95%, and from about 2% to about 15%, more preferably from about 5% toabout 10%, of high surface area cellulose. The high surface areacellulosic material used herein will typically have a surface area of atleast about 10 m² /g, preferably at least about 20 m² /g, of cellulosicmaterial. Reference can be made to U.S. Pat. No. 4,761,203, Vinson, Aug.2, 1988, incorporated herein by reference, for a thorough discussion ofexpanded cellulose fibers.

In general however, cellulosic fibers are multi-componentultrastructures made from cellulose polymers. Lignin, hemicellulose, andother components known in the art may also be present. The cellulosepolymers are aggregated laterally to form threadlike structures calledmicrofibrils. Microfibrils are reported to have diameters of about 10-20nm, and are observable with an electron microscope. Microfibrilsfrequently exist in the form of small bundles known as macrofibrils.Macrofibrils can be characterized as a plurality of microfibrils whichare laterally aggregated to form a threadlike structure which is largerin diameter than a microfibril, but substantially smaller than acellulosic fiber. In general, a cellulosic fiber is made up of arelatively thin primary wall, and a relatively thick secondary wall. Theprimary wall, a thin, net-like covering located at the outer surface ofthe fiber, is principally formed from microfibrils. The bulk of thefiber wall, i.e., the secondary wall, is formed from a combination ofmicrofibrils and macrofibrils. See Pulp and Paper Manufacture, Vol. 1,Properties of Fibrous Raw Materials and Their Preparation For Pulping,ed. by Dr. Michael Kocurek, Chapter VI, "Ultrastructure and Chemistry",pp 35-44, published joint by Canadian Pulp and Paper Industry (Montreal)and Technical Association of the Pulp and Paper Industry (Atlanta), 3rded., 1983. Expanded cellulose fibers thus refers to microfibrils andmacrofibrils which have been substantially separated from ordisassociated from a cellulosic fiber ultrastructure.

High surface area cellulose can also be made from cellulosic fibers bypassing a liquid suspension of cellulose fibers through a small diameterorifice, in which the suspension is subjected to a pressure drop of atleast 3000 psig and a high velocity shearing action, followed by a highvelocity decelerating impact. Passage of the suspension through theorifice is repeated until a substantially stable suspension is obtained.See U.S. Pat. No. 4,483,743, Turbak et al., Nov. 20, 1984, incorporatedherein by reference.

A preferred process for preparing expanded cellulose fibers is disclosedin the Vinson patent (ibid.), and involves impacting a fibrous materialhaving a fibrillar ultrastructure (e.g., cellulose fibers) with finemedia to cause microfibrils and macrofibrils to separate from saidfibrous material ultrastructure.

The length of the high surface area cellulosic material preferablyranges from about 20 to about 200 μm.

Typically, for wetlaying, the high surface area cellulose is provided asa damp pulp, generally at 15-17% solids, and preferably diluted to lessthan 4% solids content and processed in a beater or disc refiner tobreak up entanglements. The high surface area cellulose is then wellmixed with the stiffened fibers in slurry and the slurry is wetlaid asdescribed above. A blender, a deflaker or a refiner (e.g., single, cone,or double, disk refiner, or other equipment known in the art can be usedto mix the stiffened fibers and high surface area cellulose. Preferably,fine mesh wires (e.g., 84M, (84×76, 5 shed weave)) are used for improvedretention of the high surface are cellulose, rather than the more openwire conventionally used for the forming wire.

Other binding means for increasing physical integrity of theacquisition/distribution layer and/or facilitating processing of webs,especially wetlaid webs, for use as the acquisition/distribution layerinclude chemical additives, such as resinous binders, latex, and starchknown in the art for providing increased integrity to fibrous webs.Suitable resinous binders include those which are known for theirability to provide wet strength in paper structures, such as can befound in TAPPI monograph series No. 29, Wet Strength in Paper andPaperboard, Technical Association of the Pulp and Paper Industry (NewYork, 1965), incorporated herein by reference. Suitable resins includepolyamide-epichlorohydrin and polyacrylamide resins. Other resinsfinding utility in this invention are urea formaldehyde and melamineformaldehyde resins. The more common functional groups of thesepolyfunctional resins are nitrogen containing groups such as aminogroups and methylol groups attached to nitrogen. Polyethylenimine typeresins may also find utility in the present invention.

Starch, particularly cationic, modified starches may also find utilityas chemical additives in the present invention. Such cationic starchmaterials, generally modified with nitrogen containing groups such asamino groups and methylol groups attached to nitrogen, may be obtainedfrom Natural Starch and Chemical Corporation, located in Bridgewater,N.J. Other suitable binders include, but are not limited to, polyacrylicacid polyvinyl acetate.

The level of chemical additive binders which are added will typically befrom about 0.25% to about 2%, total web weight basis. Chemical additivebinders which are hydrophilic, however, can be utilized in quantities.If the chemical binder additives are added to the stiffened fibers inaqueous slurry, conventionally, nonstiffened cellulosic fibers or highsurface area cellulose is preferably also present, to enhance retentionof the chemical additive binder. Chemical additive binders can beapplied to dried or undried webs by printing, spraying, or other methodsknown in the art.

Thermoplastic Reinforced Acquisition/Distribution Layer

In another embodiment, the acquisition/distribution layer comprises anairlaid or wetlaid, preferably airlaid, web of stiffened cellulosicfibers wherein the web is reinforced with from about 10% to about 50%,preferably from about 25% to about 45%, more preferably from about 30%to about 45%, of a thermoplastic binding material, wherein thethermoplastic binding material provides bond sites at intersections ofthe stiffened cellulosic fibers. Such thermally bonded webs can, ingeneral, be made by forming a web comprising the stiffened cellulosicfibers and thermoplastic fibers, which are preferably evenly distributedthroughout. The web can be formed by either airlaying or wetlayingprocesses. Once formed, the web is thermally bonded by heating the webuntil the thermoplastic fibers melt. Upon melting, at least a portion ofthe thermoplastic material will migrate to intersections of thestiffened cellulosic fibers due to interfiber capillary gradients. Theseintersections become bond sites for the thermoplastic material. The webis then cooled and migrated thermoplastic material bonds the stiffenedcellulosic fibers together at the bond sites. Melting and migration ofthe thermoplastic material to the stiffened cellulosic fiberintersections has the effect of increasing average pore size of the web,while maintaining the density and basis weight of the web as originallyformed. This can improve distribution properties of theacquisition/distribution layer upon initial discharges due to improvedfluid permeability, and upon subsequent discharges, due to the combinedability of the stiffened fibers to retain their stiffness upon wettingand the ability of the thermoplastic to remain bonded at the fiberintersection upon wetting and upon wet compression. In net, thethermally bonded web retains its original overall volume, but thevolumetric regions previously occupied by thermoplastic fibrous materialbecomes open to thereby increase average interfiber capillary pore size.

Thermally bonded, thermoplastic-reinforced absorbent webs, utilizingconventional, unstiffened cellulosic fibers, are described in U.S. Pat.No. 4,590,114, D. C. Holtman, issued May 20, 1986, incorporated byreference herein and by Peter G. Bither in "Thermally Bonded Cores AddValue to Absorbent Products," Nonwovens World, November 1988, pp 49-55,both incorporated herein by reference. The processing techniques appliedto make those are applicable herein.

The thermoplastic binding material should be evenly distributedthroughout the web. Subsequent to formation of a dry web, the web can beheated to a temperature to melt the thermoplastic fibers but not char orotherwise damage the stiffened cellulosic fibers. Upon cooling, at leastsome of the resolidified thermoplastic material will provide bond siteswhich secure stiffened, cellulosic fibers to one another at points ofindividual fiber intersections to form a stabilizing network ofinterfiber bond sides at the intersection of the stiffened cellulosicfibers.

The thermoplastic binding materials useful for theacquisition/distribution layers herein include any thermoplastic polymerwhich can be melted at temperatures which will not extensively damagethe cellulosic fibers. Preferably, the melting point of thethermoplastic binding material will be less than about (135° C.),preferably between about 75° C. and about 175° C. In any case, themelting point should be no lower than temperatures at which the articlesof this invention are likely to be stored, whereby melting point will betypically no lower than about 50° C.

The thermoplastic binding material may, for example, be polyethylene,polypropylene, polyester, polyvinylchloride, polyvinylidene chloride.Other synthetic fibrous materials which can be utilized in thermallybonded webs are described above.

Preferably, the thermoplastic will preferably not significantly imbibeor absorb aqueous fluid. However, the surface of the thermoplasticmaterial can be hydrophilic or hydrophobic. (As used herein, the terms"hydrophilic" and "hydrophobic" shall refer to the extent to which thesurfaces are wetted by water.) The surface of the thermoplastic can berendered hydrophilic by treatment of a hydrophobic thermoplastic bindingmaterial with a surfactant, such as a non-ionic or anionic surfactant,as by spraying the material with a surfactant or by dipping the materialinto the surfactant. Upon melting and resolidification, the surfactantwill tend to remain at the surfaces of the thermoplastic. Suitablesurfactants include non-ionic surfactants such as BriJ 76 manufacturedby ICI Americas, Inc. of Wilmington, Del. and the various materials soldunder the Pegosperse trademark by Glyco Chemical, Inc. of Greenwich,Conn. Anionic surfactants can be also used. Surfactants are applied tothe fibers at a level of from about 0.2 to about 1 gram per square meterof thermoplastic binding material. Hydrophilic materials become moredesirable at higher thermoplastic material levels, particularly aboveabout 40% of the dry web.

Thermoplastic fibers for use herein can be on the order of about 0.1 cmto about 6 cm long, preferably from about 0.3 cm to about 3.0 cm.

A preferred type of thermoplastic fibrous material is commercially knownand available as PULPEX™ (Hercules, Inc., Wilmington, Del. USA). PULPEXis a polyolefin material having a very high surface area to mass ratio,which, in general, is made by spraying molten polymer and gas through anozzle into a vacuum. PULPEX is available in both polyethylene andpolypropylene forms.

The thermoplastic used can be hydrophilic or hydrophobic.

As described above, thermoplastic binder-reinforced webs of stiffenedcellulosic fibers can be made by wetlaying or airlaying processes.Airlaid webs can be made by intermixing the cellulosic and thermoplasticfibers and then airlaying according to the techniques described above.The stiffened cellulosic fibers and thermoplastic fibers can beintermixed, in an airlaid context, by carding or by metering air streamsof the stiffened fibers and thermoplastic fibrous material together anddirecting the combined system through a brush screen depositionapparatus, or other web forming device. Such techniques are known in theart. Suitable equipment includes air forming systems available from DanWebforming International Ltd. (Risskov, Denmark). A suitable method andapparatus for mixing cellulosic and thermoplastic fibers for subsequentairlaying are also described in U.S. Pat. No. 4,590,114, Holtman, D. C.,issued May 20, 1986, incorporated herein by reference. In wetlayingcontexts, the thermoplastic fibrous material can be Intermixed with thestiffened cellulosic fibers in the aqueous slurry prior to webformation.

The thermoplastic is preferably melted by through-air bonding, howeverother methods such as infra red light, etc. are not meant to beexcluded. In another variation, the web is subjected to by heatembossing on one or both faces of the web. This technique is describedin further detail in U.S. Pat. No. 4,590,114, which was previouslyincorporated into this specification.

As discussed previously, scrims such as tissue sheets and other waterpervious nonwoven sheets can be used as external support in addition toor in place of the binding means described above.

Storage Layer

A second essential element of the absorbent core is a lower fluidstorage layer which comprises at least 15%, by weight, preferably atleast 25%, of superabsorbent material (defined more fully hereafter),and from 0% to about 85%, preferably less than about 75%, of asuperabsorbent material carrier means. The principal function of thefluid storage layer is to absorb discharged body fluid from the upperacquisition/distribution layer and retain such fluid under the pressuresencountered as a result of the wearer's movements. Thus, the storagelayer is subjacent to and in fluid communication with theacquisition/distribution layer. Ideally the fluid storage layer willdrain the upper layer of much of its acquired fluid load.

As indicated hereinbefore, the storage layer comprises superabsorbentmaterial such as, but not necessarily limited to, discrete particles ofabsorbent gelling material and superabsorbent fibrous material such asacrylate grafted fibers and superabsorbent modified fibers. Thesuperabsorbent material can be in any form which can be incorporatedinto a flexible web or sheet to form the storage layer. Superabsorbentmaterials are described in more detail below. The superabsorbentmaterial, upon contact with fluids such as water or body fluids, absorbsuch fluids. (As used herein, the term "fluids" shall refer to liquids,as opposed to gases.) In this manner, fluid discharged into theacquisition/distribution layer and transported to the storage layer canbe acquired and held by the superabsorbent material, thereby providingthe articles herein with enhanced absorbent capacity and/or improvedfluid retention performance.

The superabsorbent materials intended to be encompassed in thisinvention are those which are capable of absorbing at least about 10grams, preferably at least about 15 g, more preferably at least about 20g, of Synthetic Urine (SU--1.0% NaCl aqueous solution) per gram ofsuperabsorbent material, as determined according to the hereinafterdescribed Absorbent Capacity procedures.

The superabsorbent material utilized herein is typically in the form ofdiscrete particles of absorbent gelling material. These particles willtypically be distributed within a web of fibrous material as carriermeans. The superabsorbent fibrous material can comprise synthetic ornatural fibers. Suitable fibrous carrier means are cellulose fibers, inthe form of fluff, such as is conventionally utilized in absorbentcores. Modified cellulose fibers such as the stiffened cellulose fibersdescribed above can also be used, but are preferably not used, in thestorage layer. Synthetic fibers can also be used and include those madeof cellulose acetate, polyvinyl fluoride, polyvinylidene chloride,acrylics (such as Orlon), polyvinyl acetate, non-soluble polyvinylalcohol, polyethylene, polypropylene, polyamides (such as nylon),polyesters, bicomponent fibers, tricomponent fibers, mixtures thereofand the like. Preferred synthetic fibers have a denier of from about 3denier per filament to about 25 denier per filament, more preferablyfrom about 5 denier per filament to about 16 denier per filament. Alsopreferably, the fiber surfaces are hydrophilic or are treated to behydrophilic.

The average dry density of the fluid storage layer comprisingnonsuperabsorbent fibers as superabsorbent material carrier means willgenerally be in the range of from about 0.06 to about 0.5 g/cm³, andmore preferably within the range of from about 0.10 to about 0.4 g/cm³,even more preferably from about 0.15 to about 0.3 g/cm³, most preferablyfrom about 0.15 to about 0.25 g/cm³. Typically the basis weight of thelower fluid storage layer can range from about 0.02 to 0.12 g/cm², morepreferably from about 0.04 to 0.08 g/cm², most preferably from about0.05 to 0.07 g/cm².

As with the acquisition/distribution layer, density and basis weightneed not be uniform throughout the storage layer. The storage layer cancontain regions of relatively higher and relatively lower density andbasis weight. Also as with the acquisition/distribution layer, densityvalues for the storage layer are calculated from basis weight and layercaliper measured under a confining pressure of 0.2 psi (1.43 kPa).Density and basis weight values include the weight of the superabsorbentmaterial. Additionally, the storage layer can have a superabsorbentmaterial gradient, such as with more superabsorbent material beingpresent in regions of relatively high fluid handling requirements (i.e.,near the region of fluid discharge) and less superabsorbent material atlower demand regions.

The superabsorbent material which is employed in the storage layer ofthe absorbent core will most often comprise a substantiallywater-insoluble, slightly cross-linked, partially neutralized, polymericabsorbent gelling material. This material forms a hydrogel upon contactwith water. Such polymer materials can be prepared from polymerizable,unsaturated, acid-containing monomers. Suitable unsaturated acidicmonomers for use in preparing the polymeric gelling material used inthis invention include those listed in Brandt/Goldman/Inglin; U.S. Pat.No. 4,654,039, Issued Mar. 31, 1987, and reissued as U.S. Pat. No. Re.32,649 on Apr. 19, 1988, both incorporated herein by reference.Preferred monomers include acrylic acid, methacrylic acid, and2-acrylamido-2-methyl propane sulfonic acid. Acrylic acid itself isespecially preferred for preparation of the polymeric gelling agentmaterial.

The polymeric component formed from unsaturated, acid-containingmonomers may be grafted on to other types of polymer moieties such asstarch or cellulose. Polyacrylate grafted starch materials of this typeare also especially preferred.

Preferred polymeric absorbent gelling materials which can be preparedfrom conventional types of monomers include hydrolyzed acrylonitrilegrafted starch, polyacrylate grafted starch, polyacrylates, maleicanhydride-based copolymers and combinations thereof. Especiallypreferred are the polyacrylates and polyacrylate grafted starch.

Whatever the nature of the basic polymer components of thehydrogel-forming polymeric absorbent gelling material particles used inboth layers of the absorbent cores herein, such materials will ingeneral be slightly cross-linked. Cross-linking serves to render thehydrogel-forming polymer gelling agents used in this inventionsubstantially water-insoluble, and cross-linking thus in part determinesthe gel volume and extractable polymer characteristics of the hydrogelsformed from the polymeric gelling agents employed. Suitablecross-linking agents are well known in the art and include, for example,those described in greater detail in Masuda et al.; U.S. Pat. No.4,076,663; Issued Feb. 28, 1978, incorporated herein by reference.Preferred cross-linking agents are the di- or polyesters of unsaturatedmono- or polycarboxylic acids with polyols, the bisacrylamides and thedi- or triallyl amines. Other preferred cross-linking agents areN,N'-methylenebisacrylamide, trimethylol propane triacrylate andtriallyl amine. The cross-linking agent can generally constitute fromabout 0.001 mole percent to 5 mole percent of the resulting hydrogel-forming polymer material. More preferably, the cross-linking agent willconstitute from about 0.01 mole percent to 3 mole percent of thehydrogel-forming polymeric gelling material particles used herein.

The slightly cross-linked, hydrogel-forming polymeric gelling materialparticles which may be used in the articles of the present invention aregenerally employed in their partially neutralized form. For purposes ofthis invention, such materials are considered partially neutralized whenat least 25 mole percent, and preferably at least 50 mole percent ofmonomers used to form the polymer are acid group-containing monomerswhich have been neutralized with a salt-forming cation. Suitablesalt-forming cations include alkali metal, ammonium, substitutedammonium and amines. This percentage of the total monomers utilizedwhich are neutralized acid group-containing monomers is referred toherein as the "degree of neutralization."

Webs comprising absorbent gelling material particles andnonsuperabsorbent fibrous carrier means will typically have from about10% to about 80%, more typically from about 20% to about 75%, polymericgelling material and from about 20% to about 90%, more typically fromabout 25% to about 80%, carrier means. Such webs will typically be madeby airlaying, wherein an airstream of the absorbent gelling materialparticles is metered into an airstream of the fibrous carrier means.

It is also contemplated to provide a storage layer wherein particles ofabsorbent gelling material are laminated between two or more webs offibrous material, such as exemplified in U.S. Pat. No. 4,578,068, Krameret al., issued Mar. 25, 1986, incorporated herein by reference.

As discussed above, superabsorbent fibers can be used instead ofparticles of absorbent gelling material. Superabsorbent fibers have beenpreviously disclosed in the art. Superabsorbent fibers are described inTextile Science and Technology, Volume 7, Pronoy K. Chatterjee, editor,Elsevier Science Publishers B.V. (The Netherlands), 1985, in ChaptersVII and VIII (collectively pages 217-280), incorporated by referenceherein. Synthetic and modified natural fibers, such as cellulosicfibers, can be used. The superabsorbent fibers for use herein shouldhave an absorbent capacity of at least about 10 g Synthetic Urine per gsuperabsorbent material (dry weight basis), preferably at least about 15g/g.

One type of superabsorbent fiber comprise the polycarboxylatepolymer-modified cellulosic fibrous pulps such as mildly hydrolyzedmethyl acrylate-grafted softwood kraft pulps. These superabsorbentfibers are described in U.S. Ser. No. 07/378,154, filed Jul. 11, 1989,titled "Absorbent Paper Comprising Polymer-Modified Fibrous Pulps andWet-Laying Process for the Production Thereof," by Larry N. Mackey andS. Ebrahim Seyed-Rezai, incorporated herein by reference.

Other types of superabsorbent fibers can include crosslinkedcarboxymethyl cellulose and polymer grafted cellulose fibers. Polymergrafted cellulose fibers include hydrolyzed polyacrylonitrile,polyacrylic esters, and polyacrylic and polymethacrylic acids. Thesesuperabsorbent fibers including discussion of and references toprocesses for making them, can be found in the Chatterjee's Vol. 7 ofTextile Science and Technology as previously incorporated herein byreference, include: A. H. Zahran, et al., "Radiation Grafting of Acrylicand Methacrylic Acid to Cellulose Fibers to Impart High Water Sorbency",J. of App. Polymer Science, Vol. 25, 535-542 (1980), which discussesradiation grafting of methacrylic acid and acrylic acid to cellulosefibers, as the title suggests; U.S. Pat. No. 4,036,588, J. L. Williams,et al., issued Jul. 19, 1977, which describes the graft copolymerizationof a vinyl monomer containing a hydrophilic group ontocellulose-containing material, e.g., rayon yarn; U.S. Pat. No.3,838,077, H. W. Hoftiezer, et al., issued Sep. 24, 1974, whichdiscloses polyacrylonitrile-grafted cellulose fibers.

The superabsorbent fibers can be incorporated into webs of conventionalor other nonsuperabsorbent fibers, such as in wet-laid webs as describedabove or in air-laid webs, and can also be formed into nonwoven sheets.

In another embodiment hereof, the storage layer comprises superabsorbentfibers which are formed into nonwoven sheets. Such sheets can consistessentially of superabsorbent fibers with substantially zero percentcarrier means, although such sheets can include carrier means, and suchembodiments are not meant to be excluded. Nonwoven sheets made fromsuperabsorbent fibers such as the non-acrylate superabsorbentmicrofibers and superabsorbent fibers useful for making such sheets areavailable from Arco Chemical Co. (Newtown Square, Pa., USA), under thetradename FIBERSORB™ and from Japan Exlan Co., Ltd. (Osaka, Japan) whichmarkets superabsorbent fibers comprising a polyacrylonitrile core with apolyacrylic acid/polyammonium acrylate skin under the tradenameLANSEAL™.

The storage layer embodiments of the absorbent core wherein an airlaidweb comprises the carrier means can be formed by air-laying asubstantially dry mixture of fibers and absorbent gelling materialparticles and, if desired or necessary, densifying the resulting web.Such a procedure is in general described more fully in the hereinbeforereferenced Weisman and Goldman; U.S. Pat. No. 4,610,678; Issued Sep. 9,1986. Superabsorbent fibers can be airlaid with fibrous carrier meansaccording to conventional airlaid web-forming processes. Thesuperabsorbent fibers and fibrous carrier means can be blended by, forexample, carding or Rando web formation.

Within the storage layer of the absorbent core, the superabsorbentmaterial can be uniformly distributed. Alternately, there may be regionsor zones of the storage layer which have higher concentrations ofsuperabsorbent material than do other regions or zones of the layers.

As discussed above, the acquisition/distribution layer of the absorbentcore preferably has a smaller surface area (in an unfoldedconfiguration) than the storage layer and, in fact, can have a surfacearea that is substantially smaller than, or equal to or greater than,the fluid storage layer. Generally, the surface area of theacquisition/distribution layer will range from about 25% to about 100%,preferably from about 30% to about 95%, more preferably less than about90%, most preferably less than about 85%, of the surface area of thestorage layer.

In accordance with the present invention, the acquisition/distributionlayer of the absorbent core should be placed in a specific positionalrelationship with respect to the topsheet and the storage layer of theabsorbent article. More particularly, the acquisition/distribution layerof the core must be positioned so that it is effectively located toacquire discharged body fluid and transport said fluid to other regionsof the core. Thus the acquisition/distribution layer should encompassthe vicinity of the point of discharge of body fluids. These areas wouldinclude the crotch area and, preferably for males, also the region whereurination discharges occur in the front of the diaper. For a diaper, thefront of the absorbent articles herein means the portion of theabsorbent article which is intended to be placed on the front of thewearer. Additionally, for males, it is desirable for theacquisition/distribution layer to extend to near the front waist area ofthe wearer to effectively acquire the relatively high fluid load thatoccurs in the front of the male wearer, and to compensate fordirectional variations of the discharges. The corresponding absorbentarticle regions will vary depending upon the design and fit of theabsorbent article. The acquisition/distribution layers 110 of diaper 100as shown in FIG. 2, exemplify one embodiment wherein theacquisition/distribution layer 110 is suitably positioned to receiveboth bowel and urine discharges for both males and females.

For disposable baby diaper executions, the acquisition/distributionlayer of the core is preferably positioned relative to the elongatedtopsheet and/or the storage layer such that the acquisition/distributionlayer is sufficiently enlongated to extend to areas corresponding atleast to about 50%, preferably 75%, of the length of the storage layer.The acquisition/distribution layer should have a width sufficient toacquire gushes of body fluids without direct discharge of fluid onto thestorage layer. Generally, for diapers, such as shown in FIGS. 1 and 2,the width will be at least about 5 cm, preferably at least about 6 cm.As noted, for purposes of the present invention, sections of theabsorbent article can be defined by reference to top surface areas ofthe unfolded absorbent article found in front of a given point on theline which defines the length of the absorbent article.

For purposes of determining such acquisition/distribution layerpositioning, the length of the absorbent article will be taken as thenormal longest longitudinal dimension of the elongated article backingsheet. This normal longest dimension of the elongated backing sheet canbe defined with respect to the article as it is applied to the wearer.When worn, the opposing ends of the back sheet are fastened together sothat these joined ends form a circle around the wearer's waist. Thenormal length of the backing sheet will thus be the length of the linerunning through the back sheet from a) the point on the edge of the backsheet at the middle of the wearer's back waist, through the crotch, tob) the point on the opposite edge of the backing sheet at the middle ofthe wearer's front waist. The size and shape of the topsheet willgenerally correspond substantially to the back sheet.

In the usual instance wherein the storage layer of the absorbent coregenerally defines the shape of the absorbent article, the normal lengthof the elongated article topsheet will be approached by the longestlongitudinal dimension of the storage layer of the core. However, insome applications (e.g., adult incontinence articles) wherein bulkreduction or minimum cost are important, the storage layer would nottake on the general shape of the diaper or incontinence structure.Rather the storage layer would be generally located to cover only thegenital region of the wearer and a reasonable area proximate to thegenital area. In this instance both the fluid acquisition/distributionlayer and the storage layer would be located toward the front of thearticle as defined by the topsheet such that theacquisition/distribution and storage layers would typically be found inthe front two-thirds of the article.

The storage layer of the absorbent core can be of any desired shapeconsistent with comfortable fit including, for example, circular,rectangular, trapezoidal or oblong, e.g., hourglass-shaped,dog-bone-shaped, half dog bone shaped, oval or irregularly shaped. Thisstorage layer need not be physically separated from theacquisition/distribution layer and can simply be a zone ofsuperabsorbent material concentration in a continuous web of stiffenedcellulose fiber material. More preferably, however, the storage layer ofthe absorbent core will comprise a separate web which can be used as aninsert placed underneath the acquisition/distribution layer.

The acquisition/distribution layer can also be of any desired shapeconsistent with comfortable fit and the sizing limitations discussedabove. These shapes include, for example, circular, rectangular,trapezoidal or oblong, e.g., hourglass-shaped, dog-bone-shaped, half dogbone shaped, oval or irregularly shaped. The acquisition/distributionlayer can be of similar shape or differing shape than the storage layer.

FIGS. 1 and 2 each show diaper executions embodying the presentinvention. Shown in each figure is a diaper 100 with topsheet 104 andbacksheet 102. Disposed between topsheet 104 and backsheet 102 isabsorbent core 106 having storage layer 108 and rectangularacquisition/distribution layer 110. Although not shown, storage layer108 has discrete particles of absorbent gelling material distributedthroughout.

Specifically referring to FIG. 2, the absorbent core 106 is shown ashaving a front region 112, a back region 114, and a central region 115.As previously described, the front region 112, corresponds to the end ofthe diaper 100 that would be covering the front of the wearer when thediaper was in use, and the back region 114 would be covering the back ofthe user. The absorbent core 106 of FIG. 2, specifically the storagelayer 108, has a modified hour-glass shape to provide enhanced fit andreduce in-use leakage.

FIG. 3 shows an absorbent core 106, that can be utilized in conjunctionwith a disposable diaper, having a storage layer 108 of similar shape tothose of FIGS. 1 and 2. Acquisition/distribution layer 111, however, isof a modified hour-glass shape of substantially similar shape to thestorage layer 108, though of smaller surface area.

Further with respect to FIG. 3, the absorbent core 106 has front region112, rear region 114, and central region 115. Front region 112, frontedge 117 and, at rear region 114, has rear edge 119. Front edge 117 and,has rear edge 119. Front edge 117 and rear edge 119 are connected bystorage layer side edges 122 and 123, corresponding to the centralregion 115. Acquisition/distribution layer 111 has front edge 116 in thefront region 112 and rear edge 118 in the rear region 114.Acquisition/distribution layer side edges 120 and 121, connect frontedge 116 and rear edge 118.

In preferred absorbent article embodiments, e.g., disposable absorbentdiapers, the edges 116, 118, 120, 121 of the acquisition/distributionlayer 111 will respectively be at least 0.5 cm., preferably at least1.25 cm. inside the edges 117, 119, 122, 123 of the storage layer 108,particularly in central region 115.

Superabsorbent Materials Absorbent Capacity Test Method

As discussed above, the superabsorbent materials for use in the presentinvention will preferably have an Absorbent Capacity of at least about10 g, preferably at least about 15 g, more preferably at least about 20g Synthetic Urine (1.0% NaCl aqueous solution, prepared using distilledwater) per gram dry superabsorbent material. In general, thesuperabsorbent material is place within a tea bag, immersed in an excessof Synthetic Urine for a specified time, and then centrifuged for aspecified period of time. The ratio of superabsorbent material finalweight after centrifuging minus initial weight to initial weight isAbsorbent Capacity. The following procedure can be used to determineAbsorbent Capacity. The procedure is conducted under standard laboratoryconditions.

Using a 6 cm×12 cm cutting die, the tea bag material is cut, folded inhalf lengthwise, and sealed along two sides with a T-bar heat sealer toproduce a 6 centimeter by 6 centimeter tea bag square. The tea bagmaterial utilized is grade 1234 heat sealable, obtainable from C. H.Dexter, Division of the Dexter Corp., Windsor Locks, Conn., USA, orequivalent. Lower porosity tea bag material should be used if requiredto retain fine superabsorbent materials. 0.200 grams plus or minus 0.005grams of superabsorbent material is weighed onto a weighing paper andtransferred into the tea bag, and the top (open end) of the tea bag issealed. An empty tea bag is sealed at the top and is used as a blank.Approximately 400 milliliters of Synthetic Urine are poured into a 1000milliliter beaker. The blank tea bag is submerged in the SyntheticUrine. The tea bag containing the superabsorbent material (the sampletea bag) is held horizontally to distribute the material evenlythroughout the tea bag. The tea bag is laid on the surface of theSynthetic Urine. The tea bag is allowed to wet, for a period of no morethan one minute, and then submerged and soaked for 60 minutes.Approximately 2 minutes after the first sample is submerged, a secondset of tea bags, prepared identically to the first set of blank andsuperabsorbent material-containing tea bags, is submerged and soaked for60 minutes in the same manner as the first set. After the prescribedsoak time is elapsed, for each set of tea bag samples, the tea bags arepromptly removed (with tongs) from the Synthetic Urine. The samples arethen centrifuged as described below. The centrifuge used is a DeluxDynac II Centrifuge, Fisher Model No. 05-100-26, obtainable from FisherScientific (Pittsburgh, Pa., USA), or equivalent. The centrifuge shouldbe equipped with a direct read tachometer and an electric brake. Thecentrifuge is further equipped with a cylindrical insert basket havingan approximately 2.5 inch (6.35 cm) high outer wall with an 8.435 inch(21.425 cm) outer diameter, an 7.935 inch (20.155 cm) inside diameter,and 9 rows each of approximately 106 3/32 inch (0.238 cm) diametercircular holes equally spaced around the circumference of the outerwall, and having a basket floor with six 1/4 inch (0.635 cm) diametercircular drainage holes equally spaced around the circumference of thebasket floor at a distance of 1/2 inch (1.27 cm) from the interiorsurface of the outer wall to the center of the drainage holes, orequivalent. The basket is mounted in the centrifuge so as to rotate, aswell as brake, in unison with the centrifuge. The superabsorbentmaterial-containing tea bags are positioned in the centrifuge basketwith a folded end of the tea bag in the direction of centrifuge spin.The blank tea bags are placed to either side of the corresponding sampletea bags. The superabsorbent material-containing tea bag from the secondset of tea bags must be placed opposite the superabsorbentmaterial-containing tea bags from the first set of tea bags; and thesecond blank tea bag, opposite the first blank, to balance thecentrifuge. The centrifuge is started and allowed to ramp up quickly toa stable 1,500 rpm. Once the centrifuge has been stabilized at 1,500rpm, a timer is set for 3 minutes. After 3 minutes, the centrifuge isturned off and the brake is applied. The first superabsorbentmaterial-containing tea bag and first blank tea bag are removed andweighed separately. The procedure is repeated for the second set of teabags. The absorbent capacity (ac) for each of the samples is calculatedas follows: ac=(Superabsorbent material-containing tea bag weight aftercentrifuge minus blank tea bag weight after centrifuge minus drysuperabsorbent material weight) divided by (dry superabsorbent materialweight). The Absorbent Capacity value for use herein is the averageabsorbent capacity (ac) of the two samples.

EXAMPLE I

A disposable diaper is prepared comprising a thermally bondedpolypropylene topsheet, a fluid impervious polyethylene backing sheetand a dual layer absorbent core positioned between the topsheet and thebacking sheet. The dual layer absorbent core comprises anhourglass-shaped storage layer positioned below a rectangular shapedacquisition/distribution layer, as shown in FIG. 1.

The acquisition/distribution layer comprises stiffened, twisted, curledcellulose fibers and optionally a binding means. The storage layercomprises an air-laid mixture of conventional cellulosic fluff (Foleyfluff, southern softwood kraft pulp, The Procter & Gamble Cellulose Co.,Memphis, Tenn., USA) and sodium polyacrylate polymeric absorbent gellingmaterial of the type described in U.S. Pat. No. Re. 32,649, reissuedApr. 19, 1988, and having an Absorbent Capacity of about 30 g/g. Theacquisition/distribution layer comprises a 92%/8% wetlaid mixture ofstiffened fibers and conventional nonstiffened cellulosic fibers. Thenonstiffened fibers are also made from Foley Fluff; and are refined toabout 200 CSF. The stiffened, twisted, curled cellulosic fibers are madefrom southern softwood kraft pulp (Foley fluff) and crosslinked withglutaraldehyde to the extent of about 2.5 mole percent on a dry fibercellulose anhydroglucose basis. The fibers are crosslinked according tothe "dry crosslinking process" as described above in U.S. Pat. No.4,822,453.

The stiffened fibers are similar to the fibers having thecharacteristics described in Table 1.

                  TABLE 1                                                         ______________________________________                                        Stiffened, Twisted, Curled Cellulose (STCC) Fibers                            Type = Southern softwood kraft pulp crosllinked                               with glutaralde-hyde to the extent of mole percent on                         a dry fiber cellulose anhydroglucose basis                                    Twist Count Dry = 6.8 nodes/mm                                                Twist Count Wet = 5.1 nodes/mm                                                Isopropol Alcohol Retention Value = 24%                                       Water Retention Value = 37%                                                   Curl Factor = 0.63                                                            ______________________________________                                    

The acquisition/distribution layer is a uniform, wetlaid web asdescribed in Example II. The acquisition/distribution layer has anaverage dry density of about 0.06 g/cc. An average density uponsaturation with Synthetic Urine, dry weight basis, of about 0.07 g/cc,and an average basis weight of about 0.03 g/cm². The storage layercomprises 50% by weight Foley fluff and 50% absorbent gelling materialparticles, has an average dry density of about 0.24 g/cc and an averagedry basis weight of about 0.5 g/cm².

The acquisition/distribution layer has dimensions of about 7.6 cm=22.9cm and is positioned relative to the storage layer as shown in FIG. 1.The storage layer has crotch width (at the most narrow part of thecrotch) of about 8.9 g cm, a width at the front waist area of about 21.6cm, and a width at the rear (back) waist area of about 16.5 cm.

In an alternative embodiment, the storage layer comprises about 15% ofthe absorbent gelling material particles and about 85% of Foley fluffand has a basis weight gradient such that the front 60% of the storagecore has a basis weight of about 0.11 g/cm² and a density of about 0.15g/cc and the rear 40% of the storage core has a basis weight of about0.04 g/cm² and a density of about 0.06 g/cc.

In a further embodiment, the storage core comprises about 28% of theabsorbent gelling material particles and about 72% of Foley fluff, andhas basis weight and density gradients as described immediately above.

EXAMPLE II

This example exemplifies wetlaying of a web useful for use as anacquisition/distribution layer in the present invention. The webcomprises 92% stiffened fibers, as described in Example I and Table I,and 8% highly refined Foley fluff (crill) having a freeness of about 200CSF.

A pulp slurry of the stiffened and nonstiffened fibers having a fiberconsistency of 0.1%-0.2% is pumped to a FORMAR papermaking machine at alinear velocity of 25 m/s and at rate of about 95 liters/minute. Theslurry is distributed by a fixed-roof former headbox onto an inch wide(30.5 cm) 84M, 5 shed 12 forming wire moving continuously at a rate of1.5 m/minutes. Linear velocity of the pulp slurry upon exit from theheadbox is from 50 to 100 m/s. Flow and wire movement are regulated sothat a uniform, moist sheet having a dry basis weight of about 0.03g/cm² and an average dry density of about 0.06 g/cc is formed. Sheetconsistency is increased to about 16%-22% by application of two vacuumboxes from underneath the wire, such vacuum boxes operating in sequenceat 75 mm Hg and 100 mm Hg, respectively, with a residence time for thesheet being subject to each vacuum box of about 1 second. The sheet isthen removed from the forming wire manually and dried, batchwise, in aforced convection steam heated oven for about 4 hours at about 110 ° C.

EXAMPLE III

Absorbent cores are prepared as in Example I, except that theacquisition/distribution layer is airlaid and comprises 100% of thestiffened fibers.

EXAMPLE IV

Absorbent cores are prepared as in Example III except that theacquisition/distribution layer is made from an airlaid and thermallybonded thermoplastic-reinforced web comprising 55% of the stiffenedfibers and 45% of PULPEX™ (Hercules, Inc., Wilmington, Del., USA)polyethylene microfibers having an average length of about 0.3 cm. Theacquisition/distribution layer is formed by metering airstreams of thestiffened fibers and PULPEX, and then forming the web using conventionalairlaying equipment. The web is thermally bonded by heating the web bythrough-air bonding, under unrestrained (i.e., uncompressed) conditions,and subsequently allowed to cool.

What is claimed is:
 1. An absorbent article for acquisition,distribution, and storage of bodily fluids, said article comprising:(a)a fluid pervious topsheet; (b) a fluid impervious backsheet affixed tosaid topsheet; and (c) an absorbent core disposed between said topsheetand said backsheet, said absorbent core having:(i) a fluidacquisition/distribution layer having an average dry density of lessthan about 0.30 g/cc, an average density upon saturation with 1% NaClaqueous solution, dry weight basis, of less than about 0.20 g/cc, and anaverage dry basis weight of from about 0.001 to about 0.10 g/cm², saidacquisition/distribution layer comprising from about 50% to 100%, dryweight basis, of hydrophilic chemically stiffened cellulosic fibers andfrom 0% to about 50%, dry weight basis, of a binding means for saidfibers; and(ii) a fluid storage layer, positioned beneath saidacquisition/distribution layer relative to said topsheet, comprising atleast about 15%, by weight of said storage layer, of superabsorbentmaterial and from 0% to about 85% of a carrier means for saidsuperabsorbent material;said acquisition/distribution layer having nomore than about 6.0% of superabsorbent material and having a top surfacearea which is from about 15% to about 95% of, and smaller than, the topsurface area of said fluid storage layer, said fluidacquisition/distribution layer further being maintained in unimpededfluid communication with said top surface area of said fluid storagelayer.
 2. An absorbent article as in claim 1 which is a diaper having acrotch region and wherein there is a margin from the edge of saidacquisition/distribution layer to the edge of said fluid storage layerof at least about 1.25 cm. proximate said crotch region.
 3. An absorbentarticle as in claim 1, wherein said acquisition/distribution layer issubstantially free of superabsorbent material, has a top surface areawhich is at least about 25% of the top surface area of said storagelayer and which is less than about 90% of the top surface area of thesaid storage layer, has an average density upon saturation with 1.0%NaCl aqueous solution, dry weight basis, of between about 0.02 g/cc andabout 0.15 g/cc, and has an average basis weight of between about O.01gcm² and about 0.08 g/cm², and wherein said superabsorbent material hasan Absorbent Capacity of at least about 15 g/g.
 4. An absorbent articleas in claim 3, wherein said acquisition/distribution layer comprisesfrom about 2% to about 50% of said binding means, wherein said bindingmeans comprises non-chemically stiffened cellulosic material, andwherein a fluid pervious tissue sheet is used to envelope theacquisition/distribution layer, the storage layer or both layers.
 5. Anabsorbent article as in claim 4, wherein said binding means compriseshighly refined cellulosic fibers having a freeness of less than about200 Canadian Standard Freeness, and said acquisition/distribution layercomprises from about 5% to about 15% of said highly refined fibers. 6.An absorbent article as in claim 5, wherein said carrier means for saidsuperabsorbent material comprises a web of cellulosic fibers, and saidstorage layer comprises from about 15% to about 75% of saidsuperabsorbent material, said superabsorbent material comprisingdiscrete particles of absorbent gelling material having an AbsorbentCapacity of at least about 20 g/g.
 7. An absorbent article, as in claim5, wherein said acquisition/distribution layer is a wetlaid web.
 8. Anabsorbent article as in claim 4, wherein said binding means compriseshigh surface area cellulose, and said acquisition/distribution layercomprises from about 2% to about 15% of said high surface areacellulose.
 9. An absorbent article, as in claim 8, wherein saidacquisition/distribution layer is a wetlaid web.
 10. An absorbentarticle as in claim 8, wherein said carrier means for saidsuperabsorbent material comprises a web of cellulosic fibers, and saidstorage layer comprises from about 15% to about 75% of saidsuperabsorbent material, said superabsorbent material comprisingdiscrete particles of absorbent gelling material having an AbsorbentCapacity of at least about 20 g/g.
 11. An absorbent article as in claim3, wherein said acquisition/distribution layer is an airlaid web.
 12. Anabsorbent article as in claim 11, wherein said carrier means for saidsuperabsorbent material comprises a web of cellulosic fibers, and saidstorage layer comprises from about 15% to about 75% of saidsuperabsorbent material, said superabsorbent material comprisingdiscrete particles of absorbent gelling material having an AbsorbentCapacity of at least about 20 g/g.
 13. An absorbent article as in claim3, wherein said acquisition/distribution layer is a thermally bonded webcomprising from about 10% to about 50% of thermoplastic material, saidweb being made by preparing a web of a blend of said stiffened fibersand from about 10% to about 50%, total web weight basis, ofthermoplastic material, heating the web to melt the thermoplasticmaterial, and cooling the web.
 14. An absorbent article as in claim 13,wherein said carrier means for said superabsorbent material comprises aweb of cellulosic fibers, and said storage layer comprises from about15% to about 75% of said superabsorbent material, said superabsorbentmaterial comprising discrete particles of absorbent gelling materialhaving an Absorbent Capacity of at least about 20 g/g.
 15. An absorbentarticle as in claim 3, wherein said carrier means for saidsuperabsorbent material comprises a web of cellulosic fibers, and saidstorage layer comprises from about 15% to about 75% of saidsuperabsorbent material, said superabsorbent material having anAbsorbent Capacity of at least about 15 g/g. material and said storagelayer is substantially free of chemically stiffened cellulosic fibers.16. An absorbent article as in claim 3, wherein said storage layercomprises superabsorbent fibers.
 17. An absorbent structure foracquisition, distribution, and storage of bodily fluids, said structurecomprising:(i) fluid acquisition/distribution layer having an averagedry density of less than about 0.30 g/cc, an average density uponsaturation with 1% NaCl aqueous solution, dry weight basis, of less thanabout 0.20 g/cc, and an average dry basis weight of from about 0.001 toabout 0.10 g/cm2, said acquisition/distribution layer comprising fromabout 50% to 100%, dry weight basis, of hydrophilic chemically stiffenedcellulosic fibers and from 0% to about 50%, dry weight basis, of abinding means for said fibers; and (ii) a fluid storage layer,positioned beneath said acquisition/distribution layer, comprising atleast about 15%, by weight of said storage layer, of superabsorbentmaterial and from 0% to about 85% of a carrier means for saidsuperabsorbent material;said fluid acquisition/distribution layer havingno more than about 6.0% of superabsorbent material and having a topsurface area which is from about 15% to about 95% of, and smaller than,the top surface area of said fluid storage layer, said fluidacquisition/distribution layer further being maintained in unimpededfluid communication with said top surface area of said fluid storagelayer.
 18. An absorbent structure as in claim 17 wherein there is amargin from the edge of said acquisition/distribution layer to the edgeof said fluid storage layer of at least 0.5 cm. in the regions proximateto where said bodily fluids are discharged during use of the absorbentstructure.
 19. An absorbent structure as in claim 17, wherein saidacquisition/distribution layer is substantially free of superabsorbentmaterial, has a top surface area which is at least about 25% of the topsurface area of said storage layer and which is less than about 90% ofthe top surface area of the said storage layer, has an average densityupon saturation with 1.0% NaCl aqueous solution, dry weight basis, ofbetween about 0.02 g/cc and about 0.15 g/cc, and has an average basisweight of between about 0.01g cm² and about 0.08 g/cm², and wherein saidsuperabsorbent material has an Absorbent Capacity of at least about 15g/g.
 20. An absorbent structure as in claim 19, wherein saidacquisition/distribution layer comprises from about 2% to about 50% ofsaid binding means, wherein said binding means comprises non-chemicallystiffened cellulosic material, and wherein a fluid pervious tissue sheetis used to envelope the acquisition/distribution layer, the storagelawyer or both layers.
 21. An absorbent structure as in claim 20,wherein said binding means comprises highly refined cellulosic fibershaving a freeness of less than about 200 Canadian Standard Freeness, andsaid acquisition/distribution layer comprises from about 5% to about 15%of said highly refined fibers.
 22. An absorbent structure, as in claim21, wherein said acquisition/distribution layer is a wetlaid web.
 23. Anabsorbent structure as in claim 21, wherein said carrier means for saidsuperabsorbent material comprises a web of cellulosic fibers, and saidstorage layer comprises from about 15% to about 75% of saidsuperabsorbent material, said superabsorbent material comprisingdiscrete particles of absorbent gelling material having an AbsorbentCapacity of at least about 20 g/g.
 24. An absorbent structure as inclaim 20, wherein said binding means comprises high surface areacellulose, and said acquisition/distribution layer comprises from about2% to about 15% of said high surface area cellulose.
 25. An absorbentstructure, as in claim 24, wherein said acquisition/distribution layeris a wetlaid web.
 26. An absorbent structure as in claim 24, whereinsaid carrier means for said superabsorbent material comprises a web ofcellulosic fibers, and said storage layer comprises from about 15% toabout 75% of said superabsorbent material, said superabsorbent materialcomprising discrete particles of absorbent gelling material having anAbsorbent Capacity of at least about 20 g/g.
 27. An absorbent structureas in claim 19, wherein said acquisition/distribution layer is anairlaid web that comprises from about 95% to 100% of said stiffenedfibers.
 28. An absorbent structure as in claim 27, wherein said carriermeans for said superabsorbent material comprises a web of cellulosicfibers, and said storage layer comprises from about 15% to about 75% ofsaid superabsorbent material, said superabsorbent material comprisingdiscrete particles of absorbent gelling material having an AbsorbentCapacity of at least about 20 g/g.
 29. An absorbent structure as inclaim 19, wherein said acquisition/distribution layer is a thermallybonded web comprising from about 10% to about 50% of thermoplasticmaterial, said web being made by preparing a web of a blend of saidstiffened fibers and from about 10% to about 50%, total web weightbasis, of thermoplastic material, heating the web to melt thethermoplastic material, and cooling the web.
 30. An absorbent structureas in claim 29, wherein said carrier means for said superabsorbentmaterial comprises a web of cellulosic fibers, and said storage layercomprises from about 15% to about 75% of said superabsorbent material,said superabsorbent material comprising discrete particles of absorbentgelling material having an Absorbent Capacity of at least about 20 g/g.31. An absorbent structure as in claim 19, wherein said carrier meansfor said superabsorbent material comprises a web of cellulosic fibers,and said storage layer comprises from about 15% to about 75% of saidsuperabsorbent material, said superabsorbent material having anAbsorbent Capacity of at least about 15 g/g. material and said storagelayer is substantially free of chemically stiffened cellulosic fibers.32. An absorbent structure as in claim 19, wherein said storage layercomprises superabsorbent fibers.